Switches for electric circuits are used for various fields. As conventional switches, there are known ones as proposed in, for instance, U.S. Pat. No. 4,477,704, U.S. Pat. No. 4,521,655 and so on.
FIG. 1 shows an example of a conventional switch. In FIG. 1, a reference numeral 1 designates fitting block formed by a synthetic resin, numeral 2 designates a fixed iron core formed by silicon steel plates on the fitting block, numeral 3 designates a movable iron core formed by the same silicon steel plates which is placed opposing the fixed iron core 2, numeral 4 designates an operating coil to impart a driving force to attract the movable iron core 3 to the fixed iron core 2 against a trip spring (not shown), and numeral 5 designates a cross bar having a rectangular window 2, which is formed by a synthetic resin and supports at its lower end the movable iron core 3.
Numeral 6 designates a movable contactor which is inserted in the rectangular window of the cross bar 5 and is held by a compression spring 7, numeral 6A designates movable contact provided on the movable contactor 6, a numeral 6B designates an arc horn provided at the end of the movable contactor 6, numeral 8 designates a fixed contactor provided facing the movable contactor 6 to feed an electric current, numeral 8A designates a fixed contact provided on the fixed contactor 8, and numeral 8B designates a terminal portion of the fixed contactor 8.
The arc horn 6B may be formed integrally with the movable contactor 6.
Numeral 9 designates a screw for the terminal part to connect the main body of an electromagnetic contactor to an outer electric circuit, numeral 10 designates a base to which the fixed contactor 8 is fitted, and numeral 11 designates a cover for covering the upper surface of the electromagnetic contactor, in which arc extinguishing metallic plates 13 made of a magnetic substance are provided to extinguish an arc 12 produced between the fixed contact 8A and the movable contact 6A. The arc extinguishing metallic plates 13 are arranged vertically in multi-stage, with predetermined distances, in parallel to each other and above the fixed contactor 8 so as to face the movable contactor 6.
Numeral 15 designates a commutation plate provided above the movable contactor 6. In FIG. 1, only the right portion from the center line of the switch is shown in cross-section because the right and left portions are symmetric.
The switch has the construction mentioned above. Accordingly, when the operating coil 4 of the electromagnetic contactor is rendered inactive, the movable iron core 3 is separated from the fixed iron core by means of the trip spring although it is not shown in the Figures.
Accordingly, the cross bar 5 is rendered to be a state as shown in FIG. 1, and the fixed contact 8A is separated from the movable contact 6A to thereby produce the arc 12 therebetween.
The arc 12 makes progress as shown in FIG. 2. Namely, it is attracted to the arc extinguishing metallic plates 13; it is stretched to be an arc 12A; then takes the form of an arc 12B and an arc 12C successively; the leg of the arc 12C on the arc horn 6B is transferred to the commutation plate 15; the arc 12 finally takes the form of an arc 12D; and it is cooled and extinguished by the arc extinguishing metallic plates 13.
In the conventional switch having the above-mentioned construction and operations, the arc extinguishing metallic plates 13 of the magnetic substance are positioned apart from a position where the arc is produced at the initial stage. Accordingly, there was tendency that the rising speed of an arc voltage at the initial stage 5 is low. Accordingly, the current limiting ability is poor with the result that arc energy becomes large and the breaking ability is also poor.